1. Field of the Invention
The present invention relates to a mounting device for a pressure sensor which detects the combustion pressure in an engine and more particularly to a mounting device used for mounting a pressure sensor on a cylinder head by making use of the screwed connection of a spark plug to a cylinder head.
2. Description of the Prior Art
A pressure sensor which detects the combustion pressure inside a combustion chamber surrounded by a cylinder block, a piston, and a cylinder head is used in an engine control system for controlling the ignition timing and amount of fuel injection to provide optimum combustion.
An example of a conventional mounting device for this type of pressure sensor is disclosed in Japanese Utility Model Application 63-11869 and illustrated in FIG. 1.
FIG. 1 is a is a sectional view showing the mounting conditions for a conventional pressure sensor. Illustrated in the drawing are an engine cylinder head 1, a spark plug 2 which is screwed into the cylinder head 1, and a pressure sensor 3 for detecting the combustion pressure. The pressure sensor 3 is positioned in an indentation 4 formed in the cylinder head 1 close to the mounting section of the spark plug 2. In addition, when the spark plug 2 is secrewed into the cylinder head 1, spaced by a metal gasket 5, the pressure sensor 3 is maintained in place by the application of pressure on its upper surface by a presser member 6 which projects from the gasket 5.
The combustion pressure at the pressure sensor 3 maintained in this manner is transmitted through the spark plug 2, the gasket 5, and the presser member 6 in the following manner. Specifically, because a threaded section 2a of the spark plug 2 has a spring constant which responds to the combustion pressure, when the combustion pressure is applied to the open end surface of the spark plug 2, the spark plug 2 is displaced corresponding to changes in the combustion pressure. This displacement acts on the pressure sensor 3 through the gasket 5 adn the pressure member 6. As a result, a signal corresponding to the combustion pressure is output from the pressure sensor 3 to a terminal 7.
In the conventional pressure sensor mounting device described in the foregoing, the pressure sensor 3 is secured to the cylinder head 1 by the application of pressure by the pressure member 5 provided on the gasket 5 of the spark plug 2. Therefore, the conventional pressure sensor is suitable for use in an engine such as a 4-valve DOHC (Double Over Head Cam) engine which has no excess space around the combustion chamber. On the other hand, however, the following problems are associated with this device. Specifically, because the combustion pressure is transmitted to the pressure sensor 3 through the spark plug 2 and the gasket 5, the vibrations in the spark plug 2 itself act upon the pressure sensor 3, and an accurate combustion pressure cannot be detected.
In addition, the tightening force of the spark plug 2 acts on the pressure sensor 3, so that when the tightening force of the spark plug 2 changes, the sensitivity of the pressure sensor also changes.